Ab initio MODPOT/VRDDO calculations have been carried out on RDX and α-,β-, and δ-HMX using our ab initio programs that incorporate several desirable options for calculations on large molecules. Among these are ab initio effective core model potentials (MODPOT) that permit calculations of valence electrons only explicitly, yet accurately, and a charge-conserving integral prescreening evaluation (which we named VRDDO—variable retention of diatomic differential overlap) that is especially effective for spatially extended molecules. The resultant theoretical indices are relevant to a number of properties and behaviorial characteristics of these molecules, both intra- and intermolecular. The charges on the atoms (from the gross atomic populations) are needed for calculation of the atomic multipole-atomic multipole electrostatic contributions (a major or dominant factor) to the intermolecular interaction energies. These electrostatic interaction energies are part of the input necessary for calculations on crystal packing and densities of these molecules. The total overlap populations between atoms are related to the inherent bond strengths, which are of interest in predicting and tracing initiating steps of explosive phenomena. The molecular orbital energies of the lowest unoccupied orbitals are of particular interest since these nitroexplosives have been implicated in testicular toxicity and the initial metabolic activation appears to proceed through a one-electron reduction of the nitroexplosive.